article: June 5 is World Environment Day.This makes us pay more attention to our environment and the need to protect it.
  When Wang Baoxuan,a Beijing high school boy,finishes his exercise books,he does not throw them away.He puts them in the big box in the corner of his classroom.Other students do so.Then the exercise books are sold to a paper-factory in Beijing.The paper is collected and used again by students and teachers in the school.At the same time,the money made from selling the paper goes towards schools in Inner Mongolia  for planting trees and grass.During spring time,the sandstorms often attack here.
  Wang's school is one of the schools in the capital that take part in the "green promise"environmental protection activity.So far,nearly 210,000 students have taken part in the activity,collecting more than 174 kilograms of waste paper.
  "There are some environment problems in the city,such as sandstorms.We should do our duty and encourage others to do so as well."Said Wang.
  Vice-premier Zeng Peiyan has written a letter to the teachers and students to encourage them to go on taking part in the environmental protection.
question: What is the money from selling the paper used for?
options: Buying new exercise books Helping poor students Planting trees and grass Being their pocket money